Field of the Invention
The exemplary, illustrative, technology herein relates to atomic layer deposition (ALD) systems and operating methods thereof and to gas deposition chamber and subsystem configurations that are compatible with robotic large substrate loading and unloading, with autonomous ALD coating operations, and with generating desired gas flow patterns over large rectangular substrates as well as extending the operating time between maintenance procedures so as to enhance production rates.
The technology herein has applications in the areas of producing single or multiple material monolayer coatings such as those useful for constructing Liquid Crystal Display (LCD) devices.
The Related Art
Gas, or vapor, deposition is a method for creating thin material layers (coating) on surfaces that involves exposing a solid substrate surface to a gas or vapor, hereinafter a gas, in order to deposit a thin material layer onto the solid surface, referred to herein as a “substrate”. Various gas deposition methods are known, and several are commonly used in semiconductor manufacture, for the fabrication of integrated circuits, and the like. More generally, gas deposition methods are used to form thin films onto a wide range of substrates to modify the surface properties thereof. In practice, gas deposition methods are performed by placing a solid substrate into a gas deposition chamber, also referred to herein as a “reaction chamber”, and exposing the solid substrate to one or more gases. The gases react with exposed surfaces of the solid substrate to deposit or otherwise form a new material layer thereon. Generally the material layer is formed by a chemical reaction between the gas and the substrate surface such that the film layer forms atomic bonds with the substrate surface.
Many commercial facilities are moving to add Atomic Layer Deposition (ALD) coating systems into existing material processing workflows to coat semiconductor wafers, glass substrates such as liquid crystal display substrate blanks, and the like. ALD processing methods are used to coat atomic level material monolayers onto exposed substrate surfaces using at least two gas deposition steps with each gas deposition step producing a sub-monolayer. In practice the substrate is inserted into a gas deposition, or reaction, chamber, and the chamber is evacuated to remove air, water vapor, and other contaminants therefrom before a first precursor gas is introduced into the chamber. The first precursor chemically reacts with exposed surfaces of the substrate as well as with every other exposed surface of the chamber and any other hardware surfaces and the reaction forms a first sub-monolayer. The first precursor is then flushed from the chamber and a second precursor gas is introduced. The second precursor reacts with the first sub-monolayer. The reaction of the first sub-monolayer with the second precursor gas completes the formation of a material monolayer onto the exposed substrate surfaces. The second precursor is then flushed from the chamber. Both precursor reactions are self-limiting in that once the precursor has reacted with all of the available reaction sites, the reaction stops. Accordingly ALD coatings are substantially uniform with a predictable material thickness that is substantially non-varying over the entire substrate and, depending upon cycle times, may produce uniform coating thicknesses even over the surfaces of very high aspect ratio micron-sized surface features. The second precursor reaction also creates a surface molecule that will react with the first precursor gas to form another sub-monolayer. Accordingly, the ALD process can be repeated indefinitely to build up a desired material coating thickness on the exposed substrate surfaces with a high degree of precision and purity.
Some advantages of the ALD process over various other gas deposition methods include: precise control over monolayer thickness, material coating uniformity; relatively low process temperature windows (e.g. less than 400° C.); low precursor gas consumption; high quality films; and the ability to ultimately control the total material coating thickness by controlling the number of coating cycles performed.
Some of the disadvantages of the ALD process include: a decrease in substrate throughput rate because the ALD process requires two deposition cycles per monolayer; a limited number of precursors suitable for use in coating in the ALD process and therefore a limited number of materials that can be used for ALD thin film coating; and the tendency for the ALD reactants to form coatings on every surface that is exposed to them, including the reaction chamber walls, gas flow conduits, pumps, valves, and other surfaces, and this results in continuous material buildup over time. It is a particular problem with ALD reaction chambers that ALD material layers built up on the coating chamber and other equipment over time can have adverse affects of interfering with heat transfer, flaking off and contaminating substrates, interfering with sensor readings, and damaging moving parts (e.g. in pumps, valves, and other hardware). Moreover the precursor gases tend to be highly corrosive, sometimes volatile, and usually harmful to human operators. Accordingly, precursor gases require careful handling containment. In addition, many applications require high purity precursor gasses be used to ensure that the desired electrical properties of the coating materials are not degraded by precursor contaminants, and high purity precursor gasses are expensive.
One solution to reducing precursor use is to inject low volumes of precursor gas into the chamber using precise mass flow rate control valves in the precursor gas supply line. A solution to eliminating precursor gas from the gas outflow of the deposition chamber is to install a precursor trap in the exit port. Examples of both of these devices are disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 11/167,570 filed on Jun. 27, 2005 entitled VAPOR DEPOSITION SYSTEM AND METHODS which is incorporated herein by reference in its entirety and for all purposes. While these solutions are helpful, they do not solve the problem that exposed surfaces of the ALD deposition chamber are coated during every coating cycle and periodically need to be replaced or cleaned (e.g. by an acid etch or the like).
A further problem with advancing ALD coating systems to commercially viable operation is the need to increase the chamber size as required to coat larger substrates and particularly large rectangular glass substrates as may be used for LCD displays. In particular, there are a number of engineering challenges to overcome to develop a large rectangular ALD chamber, because the chamber itself is a deep vacuum vessel capable of achieving vacuum pressures of less than 10 microtorr, and possibly less than 1 microtorr, without leaking or collapsing while also requiring a large port and associated access door for delivering a substrate into the chamber for coating. In addition, it is a challenge to handle loading and unloading of the large substrates, which are at elevated temperatures, as well as to quickly heat the chamber and substrate between load cycles and to keep the substrates free from contamination. It is a further challenge to provide a safe operating environment for users and to clean and repair the chamber without excessive impact on the overall material processing workflow.
One solution to keeping the chamber free of ALD coating layers is to install a removable and cleanable liner, such as a stainless steel liner, inside the ALD chamber to contain precursor gases and therefore prevent coating layers from forming on inside surfaces of the ALD chamber which is usually an aluminum structure due to the need for high thermal conductivity through the inside chamber walls. Such a liner can be removed from the chamber, cleaned by an acid etch or the like, and then reinstalled into the chamber. One such removable liner is disclosed in co-pending and commonly assigned U.S. Provisional Patent Application No. 61/197,948 filed on Nov. 1, 2008 entitled GAS DEPOSITION CHAMBER WITH REMOVABLE LINER which is incorporated herein by reference in its entirety and for all purposes.
A further problem with advancing ALD coating systems to commercially viable operation is the need to reduce coating cycle time and this includes the time required to load and unload a substrate, to heat the chamber and substrate to the deposition temperature, to pump the chamber to deep vacuum, to purge contaminants from the chamber, to expose the substrate to precursor gases and to flush the precursor gases out of the chamber. Applicants have found that cycle time can be reduced when the substrate is exposed to a laminar gas flow substantially over its entire coating surface and that this can be accomplished by controlling the velocity and path of input gasses by careful design of an input plenum that distributes the gas over the width of a substrate and reduces the gas velocity.
A further problem with advancing ALD coating systems to commercially viable operation is the need to fit ALD coating systems into existing production facilities where floor space is at premium. Accordingly, there is a need in the art for an ALD production coating device that provides high coating throughput in a small floor space footprint.